A caster (or castor) is an undriven wheel that is designed to be attached to the bottom of a larger object (the "vehicle") to enable that object to be moved.
The swivel caster is designed so that the wheel in the caster can rotate 360 degrees while under load. As the center hub of the wheel revolves around the center of the swivel section it is said to “cast” in that small circle. Thus, caster!
The wheel is limited in that it only spins on a single axis. It can only roll in one direction at a time. Casters incorporate wheels into their designs, but they are a bit more sophisticated. A caster includes the mounting system that holds the wheel in place.